There's no real downside to using the Tac-50 SD, it seems. You can dial in any range and the subsonic rounds will hit in the exact same place as the supersonic rounds, and still probably kill in one hit.

It's sort of a super weapon, because despite the big cloud of dust it makes, the AI are incapable of pinpointing the shooter by hearing or by muzzle flash. This makes it unique out of all snipers in the game.